If you're using Windows XP, their MovieMaker editing program is a free download (and a nice back-up for MS3). On the site, you'll also find a bunch of free clips, including a couple of countdowns.
Unfortunately, they're in, I believe, a WMV format, so I don't think you can use them directly in MS3. But, using MovieMaker, you can output them as an AVI and them have them whenever you want them.
(I'm sure there are other sources on the web also, but these countdowns are pretty nice looking and they're free.)